1. Field of the Invention
The present invention is suited for an image pickup apparatus, such as a camcorder, that effectively performs antireflection employing a fine textured structure having the antireflection function disposed at a surface (at least one of optical incident and emergent surfaces) of a lens (optical member).
2. Description of the Related Art
Typically, a lens using a light-transmitting medium (light-transmitting member), such as glass or plastic, is subjected to surface treatment, for example, antireflection coating applied to an incident or emergent surface to reduce loss of transmission of light caused by surface reflection. A known example of an antireflection film with respect to visible light is a multilayer film in which a plurality of thin dielectric films are laminated. The multilayer film is provided by formation of a thin film made of, for example, a metallic oxide on a light-transmitting substrate surface by vacuum deposition.
Nowadays, it is desired that an optical system, for example, a lens for use in a digital camera, have high optical performance and be compact and lightweight as a whole. To respond that desire, a lens that has a large diameter or a lens that includes a surface having a small radius of curvature is being widely used.
When such a lens is used in an optical system, a light ray enters the outer region of the lens at a large angle. Because of this, it is difficult for an antireflection film composed of a single thin dielectric film or of laminated multiple thin dielectric films to satisfactorily reduce reflection because an incident angle is in a wide range. This causes undesired light, such as a ghost or flare.
In view of the circumstances, Japanese Patent Laid-Open No. 2005-316386 describes an optical system including a light-transmitting member that has a concave surface facing an aperture stop and that has an antireflection film formed on the light-transmitting member, the antireflection film including at least one layer formed by sol-gel process.
The optical system disclosed in this patent document achieves a low reflectance in a wide incident-angle range and reduces the occurrence of ghosts and flares.
A known example of an antireflection structure for use in a lens is a fine textured structure that has a mean pitch shorter than the wavelength of visible light described in Japanese Patent Laid-Open Nos. 2005-157119 and 2006-10831.
A digital camera for taking an image using an image pickup element (e.g., a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS)) employs an APS-C size, which is smaller than a film size, in consideration of compatibility with an existing image taking lens in most cases.
As illustrated in FIG. 2, the dimensions of a film size (full size) 8 are approximately 24 mm long and 36 mm wide. The dimensions of an APS-C size 47 are approximately 15 mm long and 22.5 mm wide and approximately 1/1.6 of the full size.
An APS-C size camera has the same effect as in a trimming mode in image taking using a full-size film camera and has a smaller angle of view. Because of this, an image taken by an APS-C size camera is similar to that using a more telephotographic image taking lens.
The angle of view in image taking using an APS-size camera in which an interchangeable lens having the same focal length as in a film camera is attached is approximately 1.6 times that using the film camera in equivalent focal length. Accordingly, an APS-C size camera has the advantage of being able to take an image in a more telephotographic region using a telephotographic lens. However, with a standard lens (f=50 mm) or a similar one of a film camera or a wide-angle lens, an APS-C size camera has a smaller angle of view. Thus, an APS-C size camera needs a wider-angle image taking lens.
A typical standard zoom lens for a full-size camera has a focal length of the order of 28 mm at a wide-angle end, whereas an APS-C size camera needs a lens having a focal length of the order of 17 mm at a wide-angle end. Therefore, as a wide-angle lens for use in an APS-C size camera, a super wide-angle image taking lens, such as a lens having the 14 mm focal length or that having the 15 mm focal length, is being employed.
For these focal lengths, the angle of view diagonal at both sides in a film camera is very wide at approximately 110°. Accordingly, in a film camera, the outer diameter (effective diameter) of a lens element increases from the aperture stop toward the front. In particular, a front lens element has a spherical shape that has the center of curvature in the aperture stop or its vicinity and that has a larger angular aperture.
FIG. 24 is a cross-sectional view that illustrates an example of a wide-angle lens as an interchangeable lens for use in a single-lens reflex camera. As illustrated in FIG. 24, light rays to image heights pass through separate regions in the lens element nearest to the object.
In FIG. 24, lens elements 13 to 19, a variable stop 21, and a flare-cut stop 27 are illustrated.
FIG. 22 illustrates a range of effective light rays that pass through a lens surface of a single lens disposed between the object and the aperture stop 21.
In FIG. 22, reference numeral 48 depicts an effective range of light rays in the case of a full size, and reference numeral 49 depicts an effective range of light rays in the case of an APS-C size. As illustrated in FIG. 22, effective light rays for an APS-C size are represented by the substantially rectangular inner region 49 and corresponds to only a part of the lens diameter. When an APS-C size is used, light rays other than those passing through the region 49 are unnecessary. Such unnecessary light causes a ghost or flare.
Even if, in consideration of an effective diameter of light rays of an angle of view when a full size is used, a special light-shielding board 50 for flare cutting and for blocking transmission of light in the shaded region in FIG. 22 is provided in an optical path, its aperture portion has a size approximately twice the size of necessary light rays in the region 49 for an APS-C size. Accordingly, it is difficult to sufficiently avoid the occurrence of flares.
However, for a wide-angle lens illustrated in FIG. 24, the region 48 outside the region 49 illustrated in FIG. 22 is in the outer region of a lens surface G1b. Thus, the inclination of the lens surface is large, and the proportion of unnecessary light incident at more than 60° is also large.
An antireflection film including laminated multiple thin dielectric films aims to reduce reflectance by adjusting the refractive index and thickness of each film and causing reflected light occurring in a surface or interface to interfere with each other. Therefore, when the antireflection film is applied to a lens surface of an optical system, antireflection performance (antireflection function) is high at a specific wavelength or a specific incident angle.
However, there is a problem in which the antireflection performance is significantly low, that is, the wavelength range characteristic or incident angle characteristic is low at the other wavelength or the other incident angle.
In contrast, with a fine textured structure having a fine textured configuration formed on a lens surface, the antireflection characteristics having a good incident angle characteristic in a relatively wide range are readily obtainable.
However, for a wide-angle image taking lens, the incident angle of an off-axis ray to a lens surface is large. To obtain a good antireflection function (wavelength range characteristic) in a wide wavelength range and obtain a good antireflection function (incident angle characteristic) in a wide incident angle range, it is important to appropriately set application of a fine textured structure to a lens surface.
If a fine textured structure is applied to an inappropriate lens surface location in an image taking lens having a wide angle of view, a good antireflection effect is not obtainable. This causes many flares or ghosts, and it is difficult to acquire a high-quality image.